Warren



Jan. A3l, 1956 c. w. WARREN 2,733,357

CURRENT LIMITING POWER SUPPLY FOR PHOTOMULTIPLIER Filed Oct. 5 1951 4Sheets-Sheet l ATTORNEY Jan. 3l, 1956 c. w. WARREN 2,733,357

CURRENT LIMITING POWER SUPPLY FOR PHOTOMULTIPLIER 4 Sheets-Sheet 2 FiledOct. 5, 1951 lNvENToR @H4/@z f5 MA WAP/PI/V;

ATTORNEY C. W. WARREN l Jan. 31, 1956 CURRENT LIMITING POWER SUPPLY FORPHOTOMULTIPLIER 4 Sheets-Sheet 3 Filed 0G13. 5, 1951 lNvENToR c/qAz J W.V4/Ping ATTORNEY Jan. 31, 1956 c. w. WARREN 2,733,357

CURRENT LIMITING POWER SUPPLY FOR PHOTOMULTIPLIER Filed Oct. 5, 195] 4Sheets-Sheet 4.

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7'07-4L @yA/O05 l/LTGE INVENTOR C/JZES W. WF/V,

ATTORNEY and condensers.

2,733,352' Patented dan. 31, 1956 ice CURRENT LIMITING POWER SUPPLY FORPHOTOMULTIPLIER Charles W. Warren, North Plainfield, N. J., assignor toAmerican Cyanamid Company, New `York, N. Y., a corporation of MaineApplication October 5, 1951, Serial No. 249,863 3 Claims. (Cl. Z50-207)plication made possible by the dynodes, however, imposes a seriousproblem. Frequently, an instrument using a photomultiplier tube withhigh sensitivity of extremely low light levels may accidentally beexposed to much higher light levels, for example, when the machine isopened to change samples or other elements the light emission orreilectance of Which is to be measured. It is of course possible toguard against such occurrences by a complicated machine involving lightlocks but usually the operator is relied upon to cut oic the voltage onthe photomultiplier anodes when the top is to be subjected to a moreintense light. This, however, puts the protection of the photomultipliertube entirely in human and, therefore, fallible hands. Excessive lighton the photomultiplier tube can soon damage the tube, for example, byreducing the emission eliiciency of the cathode. Not only is thesensitivity of the photomultiplier tube damaged cuits to recalibrate thewhole machine for it is impossible with enormous multiplication of themulti-stage multiplier to make two tubes that are exactly alike. Theloss in time and maintenance labor involved in recalibration is often aneven more serious loss than the cost of the damaged photomultipliertubes, for often a machine costing many thousands of dollars is kept outof use. Also in some instances Where specially calibrated scales areused for direct reading purposes, it may be necessary to prepare a newscale when tubes are changed.

The present invention depends on an automatic control of the dynodevoltages of the photomultiplier tube when the anode current exceeds acertain predetermined ligure well within the safe current carryingcapacity of the tube. Sharp lowering of the voltage applied to thedynodes of the photomultiplier enormously decreases its sensitivity andthus maintains a maximum current well within the safe limits of thetubes even though the photomultiplier cathode may be subjected to'relatively strong illumination, many orders of magnitude greater thanthe low level illumination at which the tube normally operates at fullsensitivity.

The power output of the radio frequency oscillator is easily controlledby applying a controlled voltage to the oscillator tube. triodeoscillator tubes, the controlled voltage is prefer- In the case of` ablythe plate voltage whereas in pentode oscillator tubes, a more sensitivecontrol is effected by controlling the voltage on the screen grid.

The control voltage is obtained very simply from the voltage drop in aresistance in the supply circuit of the photomultiplier tube. a signalto control a tube connected as a Variable resistor. The tube in turncontrols the voltage on one of the elements of the oscillator, the platein the triode version and screen of the pentode.

The control tube may be used without other elements and this will resultin a satisfactory protection of photomultiplier tubes. However, withinthe working range of the plate currents in the photomultiplier tubethere Will be some changes in the voltages on the dynode. While thesechanges are small compared to the great change only the signal but alsothe characteristics of the control tube. For some purposes the errorintroduced by changing voltages is either of no importance or can be thecontrol tube drops the voltage to the point Where the voltage regulatortube 1s extinguished, control of tne the photomultiplier output.

The invention will be illustrated in greater detail in connection withtwo typical control circuits which are shown in the drawings in which:

Fig. l is a schematic diagram of a control circuit using a pentodeoscillator with screen grid voltage control;

using a triode oscillator varying plate voltages;

Fig. 3 is a schematic diagram using a control tube associated with avoltage regulator; and

Fig. 4 is a graph of regulation in a typical circuit with various dynodevoltages.

in Fig. l, the photomultiplier tube is shown at l. Only the circuits ofthe photomultiplier tube are shown as the nature of the instrumentreceiving the signal from the photomultiplier tube is not concerned inthe regulatory circuits of the present invention. Therefore, the signalfrom the final anode 2 is simply shown as passing through a directcurrent stopping capacitor 3 to whatever device is controlled oractuated by the photomultiplier signal.

manner using This voltage drop is then used as lparallel with thevoltage regulator tube 1 5.

Voltage for the photornultiplier tube is obtained from GAKS pentode 7,operating as a radio frequency oscillator using the tunable tank circuit6 in its plate supply. The grid circuit including the coil 8 is ofconventional design. All capacities are expressed in microfarads (ai).The plate circuit 6 forms the primary of a transformer, the secondary ofwhich 9 feeds a IZ?. high voltage rectifier. The high voltage aftersuitable filtering is applied to the multiplier tube, the anode beingnear ground potential with the cathode at high negative potential.

The control circuit of the present invention includes a 250 K resistor1t) to ground with a by-pass condenser 11 which short circuits thecontrol resistor at signal frequencies. The high potential end of theresistor 1) connects through a suitable filter resistor l?. to the gridof the control tube 12, which is shown with the 6 AK6 connected as aself-biased triode. A. C, components are by-passed to ground by theconventional condenser. The anode of tube and the resistor 14 may beconsidered as constituting a voltage divider in which the tube opcratesas a variable resistor depending on the voltage applied to its controlgrid. Since the screen grid of the oscillator tube is connected at apoint where the tube joins the resistor its voltage will vary as thetube resistance changes. For smooth control it is often desirable tovary the voltage on the screen grid nonlinearly with voltage on the gridof the tube 13. This is easily effected by a suitable biasing resistorin the cathode circuit.

ln operation, the resistor 14 is adjusted under conditions ot noillumination of the photomultiplier tube until the voltage on the screenof the oscillator 7 has a suitable value to provide the desired highvoltage on the anode 2 of the photomultiplier tube. Under lowillumination, the voltage drop through the load resistor 4 and controlresistor 10 is comparatively small. When, however, excessiveillumination of the photomultiplier tube cathode takes place, thevoltage on the control grid of the control tube 13 increases, this tubebegins to draw a large amount of current, The resulting voltage dropthrough the resistor 14 lowers the screen voltage of the oscillator tube7 and hence decreases oscillator output and therefore anode voltage onthe photomultiplier tube 1 until the current decreases to a safe value.The control operates with a small time delay which, however, is largecompared to signal frequency, so that the photomultiplier tube 1 cannotoperate at excessive current level for suticient time to damage it.

Fig. 2, in which the same elements bear the same reference numerals,shows a slightly dilerent design of oscillator. In this case, controltube and oscillator are two sections of a 12AU7. The operation, however,is the same. The control voltage is developed across the resistor r10.The operation as in the case of the circuit of Fig. lresults in avarying current through the control tube section 13 of the l2AU7 whichvaries the voltage drop through the resistor 14 and therefore the platevoltage on the oscillator section 7. The circuit of Fig. 2 Ais simple,requiring only a single dual purpose tube but the regulation is notquite so sharp as `in the case of the pentode oscillator in the Fig. lwhich is more sensitive to changes in control voltage.

in Fig. 3 the circuit is similar to Fig. l except that thephotomultiplier anode 2 receives B+ voltage from the ordinary powersupply and -a voltage regulator tube 15, type VRlSO, is connectedbetween the cathode and ground of the control tube. Instead ofconnecting the control grid of the oscillator vtube 7 to the junction ofa variable resistor and the anode of the control group including theregulator tube 15 the connection is to the potentiometer 16 in thecathode Vcircuit of the control tube 13 which potentiometer with anVadditional resistor is in lhe rest of the oscillator circuit and highvoltage rectifier is the same as in the preceding figures except thatthe voltage required is not as high since the anode 2 of the photomultiplier tube, instead of being connected to ground preferred.

' the tube and resistor.

beyond a predetermined point.

Vin each 4o potential, has applied to it the positive voltage of thepower supply. Accordingly, the oscillator is required to supply onlyfrom about 300400 volts between ground and cathode of thephotomultiplier tube. The circuits are adjusted principally by thesetting of the potentiometer 16 so that in the Working range of thephotomultiplier tube the voltage regulator tube 15 remains ignited.Accordingly, throughout the working range of the photomultipliervoltages on the dynodes of the photomultiplier tube remain constant.However, as soon as the illumination on the photomultiplier tube exceedsa certain predetermined limit, the current drawn by the control tube 13drops the cathode voltage to the point where the regulator tube isextinguished. The current of the control tube then tiows through thepotentiometer 16 and any further increase in current results in a veryrapid drop in voltage on the screen grid of the oscillator 7 whichreduces the voltage applied to the photomultiplier dynodes.

Fig. 4 is a graph of a typical operation of the circuit of Fig. 3 tordifferent voltages on the dynodes. ln each case the circuit elementshave been adjusted so that the voltage regulator tube 15 is extinguishedwhen the current flowing to the anode 2 reaches 120 microarnperes. ltwill be noted that the voltages stay absolutely constant throughout thewhole of the working range of the photomultiplier tube which rendersthis modification of this invention the preferred one where maximumaccuracy is required.

Fig. 3 shows the application of a voltage regulator tube to a circuit inwhich the oscillator tube is controlled by varying the voltage on ascreen grid of a tetrode. In exactly the same manner, the triode circuitof Fig. 2 may 'be provided with the advantages of the modification usinga voltage regulator tube.

The advantages of using the power supply voltage as part of the totalvoltage on the anode of the photomultiplier tube can also be enjoyedwith the modifications shown in Figs. l and Z even though no voltageregulator tube is used.

In the drawings, there have been shown typical circuits using Well-knownreceiver type tubes. The values of resistances and capacitors givesatisfactory regulation in the circuit shown. It is an advantage of -thepresent invention, however, -that these values are not critical andordinary `elements may be used, for example, resistors in commerciallyreadily available tolerances. Other tubes may, of course, be used withsuitable adjustment of the values of the circuit components tocorrespond to the tube characteristics.

In Figs. l and 2 the control elements of the oscillator tube, plate inthe case of triode and screen grid in the case of pentode, are shown asconnected to the point where the control tube joins the resistor to forma voltage divider. This gives the most sensitive control and is It isobvious, however, that it is not necessary that the voltage be taken otexactly at the point 4on the Vvoltage divider where the resistorconnects to the control tube. The connection may be at other points butin order to obtain adequately sensitive control, such points must be ata potential near that of the junction of This condition is villustratedin the modification shown in Fig. 3 where the screen grid of 4theoscillator tube isconnected to a point near, but not exactly at, .thejunction `of the control tube and resistor.

The control .circuit applies a positive control voltage when the`current of the photornultiplier tube increases This is by tar the mostsatisfactory arrangement and is preferred. It is obvious, of course,4that the controlled element of the control tube,

-in which case the control voltage, must become more negative withexcessive. currents.

lt will be VVnoted that thecontrol Vtube and its resistance At` the.modificationsshown vary the D. C. power input of theR. F. oscillator.In some cases a variation of the D. YC. anode `voltage 1s effected, inothers a variare parts of the total D. C. power input.

This application is in part a continuation of my prior applicationSerial No. 208,510, tiled January 30, 1951, now abandoned.

I claim:

l. A current control circuit for photomultiplier tubes comprising apower supply for the photomultiplier tube at least a portion of whichincludes a radio frequency electron tube oscillator, at least one D. C.power input to said oscillator, a resistance element in the circuitconnecting the power supply to the anode of the photornultiplier tube,filtering means between the anode and the resistance element having asuflcient time constant to eliminate alternating current fluctuations inthe resistance element, whereby fluctuations in the averagephotomultiplier anode current result in variations in potential dropacross the resistance element constituting a varying control voltage, avoltage divider in at least one D. C. power supply input to theoscillator, said divider comprising an electronic tube having a controlgrid and at least one resistance in series therewith, a gas voltageregulator tube having a tiring voltage not less than that of the D. C.power input to the oscillator for maximum photomultiplier tubesensitivity shunting at least a portion of the voltage dividerresistance, the electronic tube in the volt-4 age divider being capableof a tube current over a predeterminedy grid bias range which currentmultiplied by the portion of the voltage divider shunted by the voltageregulator tube at least equal to the tiring voltage of said tube, acircuit connection from the voltage divider to an electrode of the R. F.oscillator tube, said connection being from a point on the regulatortube shunted portion of the voltage divider resistance whereby D. C.power input to the oscillator ows from this point, connecting means fromthe control resistance in the anode circuit of the photomultiplier tubeto the control grid of the electronic tube in the voltage divider,whereby current uctuations in the resistance apply voltage to saidcontrol grid which voltage decreases with increasing anode current andhence decreases current through the voltage divider electronic tube andtherefore voltage on the regulator tube, whereby as the averagephotomultiplier anode current increases at first no changep'hotomultiplier tube anodes.

2. A circuit according to claim 1 in which the resistance shunted by theregulator tube is in the cathode circuit of the voltage dividerelectronic tube.

3. A circuit according to claim 2 in which a portion at least of thevoltage divider resistance shunted by the regulator tube is in the formof a potentiometer the connection to the oscillator electrode being tothe movable arm of said potentiometer whereby a series ofphotomultiplier tube sensitivities can be obtained by adjustment of thepotentiometer each range being controlled by the voltage regulator tubeup to the predetermined maximum photomultiplier anode current at whichthe regulator tube extinguishes.

References Cited in the file of this patent UNITED STATES PATENTS

